Current impact beams for automotive bumpers and side intrusion beams are configured such that when the impact beam is subjected to a direct impact, the kinetic energy is dissipated by the work of displacing the beam inwardly with respect to the supporting structure. In the process, kinetic energy is converted to strain energy in the resisting member. Depending on the end fixation conditions, some portion of the strain energy would be manifested as bending strain energy and the rest as membrane strain energy. Unless there is significant lateral rigidity of the end fixation or that the curvature of the beam is approaching that of an arch, which is rarely practical, the strain energy would be predominantly stored as bending strain energy. The amount of bending strain energy that can be dissipated is limited by the peak stress developed in the structure as the resisting force increases while mitigating the impact. If the impact beam is mounted with longitudinally positioned supplementary energy absorbers including hydraulic attenuators, absorptive foams, crush tubes or spring supports, then the impact beam would have to be designed to withstand at least the trigger loads of these devices.
One example, as disclosed in U.S. Pat. No. 5,269,574, includes a bumper of an `I`-type configuration with a ribbed web to provide some torsional rigidity. During a central impact, the kinetic energy of the impact is converted to predominantly bending strain energy. The distribution of the bending strain energy along the span of the beam would be similar to the distribution of the bending moment on a simply supported beam. The peak stress occurs at the impact location and decreases gradually along the length to a smaller magnitude at the ends. Only a small central portion of the beam would be strained to the elastic limit while the rest of the span would be stressed to less than the limit.
An impact beam should be designed to maximize the work done in straining the beam to the elastic limit. The external work done is the product of the resisting force and the distance traversed while resisting the load. Internal to the beam this equates to the totality of strain energy stored in the beam which is the volumetric integral of the product of the average stress and strain. It is imperative to note that a beam designed to maximize static stiffness does not necessarily translate to one that mitigates maximum kinetic energy under impact.
Another example, as disclosed in U.S. Pat. No. 4,961,603 includes a curved bumper member with a rear tension means that are directly and pivotally connected. The tension member provides a means for relieving the longitudinally extending load members, to which the bumper is mounted, of the large lateral forces that would be developed when the curved beam is impacted. Depending on the actual sweep of the curve member, the bending moment distribution along the span could be at best maximum at the center and at the ends and varying gradually in between. During a central impact, the strain energy in the bumper will be stored as bending strain energy and membrane strain energy in the beam. Depending on the relative magnitude of the bending and the axial forces, the stress could be maximum at a small span where the bending moment is maximum or at the region where the axial force is approaching the critical buckling load of the beam.
U.S. Pat. No. 1,626,347 discloses a curved bumper in which the lateral forces are countered by the bracing action of a member extending across the rail at the point of attachment. Again, here the peak stress would be at the point where the peak bending moment occurs or where the axial force approaches the critical buckling load of the beam.
Admittedly both these designs could result in a much more uniform stress distribution as the sweep approaches that of an arch. This is a result of the membrane mode of energy storage dominating with larger sweeps. However these designs do not provide a means for controlling the large amounts of membrane energy developed in the curve beam. Curved beams with large sweeps generally have the tendency to buckle, which is essentially the process where the unstable membrane strain energy exceeding the stability threshold reverts to the more stable bending strain energy.
Optimally a beam designed for impact should be no stiffer than the surrounding structure and have as uniform a stress distribution over the span of the beam as practically feasible. In addition, for work to be done, the resistant force should traverse over as large a distance as possible or internal to the beam, the stress should be uniformly sustained over as large a span as possible. The load and the stress should be maintained as the beam deforms so that the ideal square energy pulse would be obtained as the kinetic energy of the impact is mitigated.